circle.yml: Install pkg-config from brew
circle.yml: Install pkg-config from brew

`opam depext` tries to do this later but trips over an issue with brew with
messages similar to:
http://stackoverflow.com/questions/38410020/homebrew-error-update-report-should-not-be-called-directly

Signed-off-by: Ian Campbell <9be92b3dbbd6611e3e4c9209fb3d04b4919e7cca@docker.com>

djs55/vpnkit,djs55/vpnkit,djs55/vpnkit